The field of this invention relates to drillpipe structures which can accommodate the loads imposed during drilling, while at the same time facilitate making a variety of measurements while drilling.
Present drilling technology incorporates a metal tubular connecting the surface drilling equipment to the drill bit at the bottom of the well One of the difficulties in the design of measurement-while-drilling (MWD) tools is that they need to be attached to the drill string without undue encumbrances of the drilling operation. The bottom of the drill string where the MWD tools are located is usually composed of metal tubulars called collars, which have to provide mechanical integrity and sometimes weight to the drill string while conveying drilling fluid from the surface to the bit. MWD sensors are either mounted on and integral to the collar or are housed in a central package inside the bores of the collars. Certain formation sensor transducers, such as formation resistivity sensor antennas, are restricted from being located inside the collar bores because the metal walls seriously degrade their ability to measure wellbore parameters outside the collars. This inability to xe2x80x9csee throughxe2x80x9d the collar walls usually causes certain MWD tools to be more expensive to build and maintain than central xe2x80x9csondexe2x80x9d-based tools.
Antennas which are housed or attached to the various components of the drill string are used in MWD applications either to enable measurements of electrical parameters in the downhole environment or to enable communication of information with the surface or other drill string components. A challenge in making an antenna go downhole in a drilling environment requires satisfaction of both electrical and mechanical constraints. One particular type of antenna is the transverse electric type in which current flows around the drill string component in which the antenna is contained. The drill string component is usually a drill collar and the result of the current flow is to induce, in the case of a transmitting antenna, a magnetic field in the region around the antenna. In the situation of a receiving antenna, the magnetic field, which is locally axial in a direction along the drill string, induces current in the antenna element or elements which are around the drill string component in an azimuthal direction. Transmitting and receiving antennas are similar in construction, the difference being the direction of energy flow. A gap has to be maintained between the azimuthal current element and the high-conducting drill string in order for the magnetic field to encircle the element and thereby allow energy flow to or from the downhole environment proximate to the antenna.
One technique in the construction of antennas has been to neck down the highly electrically conducting drill collar in the antenna region, so that the antenna element does not extend out past the radius of the collar in order to protect it from the drilling environment. The region around the antenna element is then covered with electrical non- or semiconducting materials such as fiberglass, ceramic and rubber, to protect the element from the drilling environment. Another design is to neck down the drill collar but to use a slotted mask in place around the antenna element. The mask provides more protection from the drilling environment than other methods and the mask also provides electrical shielding necessary in these applications. The axial slots are cut in the mask to allow the magnetic fields to pass from the region inside the metal mask, where the antenna element is contained, to the region out-side so that the fields may be either received or caused in the region adjacent the drill string.
These methods involve the weakening of the drill string due to the neck down region which has been described for placement of the antenna. The presence of material in the outer diameter of the drill string is important in determining its strength, which is critical in the smaller drill collar sizes.
Composite drill type has been used as described in U.S. Pat. No. 5,332,049. This type of hybrid structure of a composite with hardened steel end fittings suffers drawbacks of failures at the juncture of the metallic and composite segments. Additionally, such designs of composites have included fibers applied in layers successively over each other, where each layer was made entirely of one kind of fiber, such as carbon or glass fiber. Successive layers were placed one over the other during construction, until the tube was complete. The layers which were made entirely of carbon fibers had the disadvantage that they prevented the passage of electromagnetic energy. Other constructions which involved composites used to provide strength to resist internal pressures are known but are unsuitable for drilling application. Some examples of such construction for cables or tubular goods employing layers of composite materials include U.S. Pat. Nos. 5,110,644, 5,234,058 and 5,172,765.
Even the composite materials which have been introduced for drilling applications are made of a hybrid glass/carbon fiber-reinforced epoxy and are not conducive to permit electromagnetic energy to exit and reenter in the azimuthal direction for facilitating MWD of such formation features as resistivity.
Accordingly, what is desired and is an objective of the present invention is to provide a drill collar structure that has sufficient structural rigidity to withstand the rigors of drilling. At the same time, the structure should be capable of supporting the MWD equipment, some types of which rely on electromagnetic energy for the measurements taken during drilling activity. Thus, the composite structure of the present invention has the objective of allowing electromagnetic energy to exit and reenter, as well as to facilitate the location and operation of other borehole property measuring equipment, so that the entire assembly functions to allow real-time data of borehole conditions while at the same time facilitating the drilling operation. These objectives have been addressed in a plurality of alternative embodiments which are designed to address the two main criteria of sufficient physical rigidity of the drill collar assembly, while at the same time the facilitation of the measurements needed during the drilling operation.
A drill collar structure to facilitate measurement-while-drilling (MWD) techniques while at the same time having sufficient rigidity to facilitate the drilling operation is disclosed. In some embodiments, a metal structure involving openings such as slots, preferably oriented longitudinally along its axis, but also in other configurations, are disclosed to allow sufficient strength while at the same time allowing exit and entrance of electromagnetic energy. A composite structure is also disclosed which, for given layers, has adjacent fibers such as glass and carbon, so that when the layers are overlapped, glass areas overlap glass areas throughout the radial thickness of the composite tube to create xe2x80x9cwindowsxe2x80x9d for the entrance and exit of electromagnetic energy. In yet other embodiments, the drill collar can be made of a metallic frame structure with a multiplicity of openings which are filled with a composite material. The metallic frame structure provides structural rigidity while the openings, filled with composite material which are attached to the metal structure, form a fluid-tight cohesive structure sufficient to withstand the rigors of drilling, while at the same time allow the measurements of the formation to be taken through the windows or by making use of sensors embedded in the windows. In another alternative composite structure, carbon fibers can be used if they are electrically insulated in the areas where electromagnetic energy is to enter and exit the structure. Another composite alternative is to alternate carbon and glass fibers in particular sequences or to use insulated carbon fibers to facilitate the operation of instruments which can be mounted in the structure whose operation could be negatively affected by conductivity in the wall in an azimuthal direction.